Phosphors using a silicate, a phosphate (for example, apatite) or an aluminate as the host material, where a transition metal or a rare earth metal is added as an activating material to the host material, are widely known. In particular, with recent practical realization of a blue LED, development of a white light source utilizing the blue LED is being aggressively pursued. A white LED is expected to have a lower power consumption and a longer service life than existing white light sources and therefore, development as a backlight for liquid crystal panels, an indoor lighting device, a backlight for automobile panels, and the like is proceeding.
The currently developed white LED is obtained by coating a YAG (yttrium.aluminum.garnet)-based phosphor on the surface of a blue LED, where a Ce-activated YAG-based phosphor converts blue light of the blue LED into yellow light. A part of blue light at a wavelength of 450 nm emitted from the blue LED penetrates the phosphor layer, and the remainder strikes the phosphor and is converted into yellow light. These two light colors of blue and yellow are mixed to make the light look white.
However, the YAG-based phosphor has a problem that not only the white light is blue-tinted due to reduction in the spectral intensity when the excitation wavelength exceeds 400 nm, but also the luminous efficiency is low because the excitation energy of the blue LED does not agree with the excitation energy of the YAG-based phosphor. There is also a problem that durability of the coated phosphor layer is insufficient. Accordingly, improvements are demanded on the performance of the phosphor itself used for wavelength conversion.
In Patent Document 1, the applicant of the present invention has disclosed an α-sialon-based oxynitride phosphor represented by the formula: MxSi12−(m+n)Al(m+n)OnN16−n:Lny (wherein 0.3≦x+y<1.5, 0<y<0.7, 0.3≦m<4.5, 0<n<2.25, and assuming that the valence of the metal M is a and the valence of the lanthanide metal Ln is b, m=ax+by), wherein a part or all of the metal M (M is at least one metal selected from Li, Ca, Mg, Y and a lanthanide metal excluding La and Ce) solid-dissolved in α-sialon is replaced by the lanthanide metal Ln (Ln is at least one lanthanide metal selected from Eu, Dy, Er, Tb and Yb) for the luminescence center.
The α-sialon-based oxynitride phosphor disclosed in Patent Document 1 is certainly an excellent phosphor, but improving the luminescence intensity of the phosphor is in demand.